How do I tell if a sales drop is seasonal or a real problem?
Short answer
Four checks, in this order. Compare the period against the same period last year rather than the one before it, which holds seasonality constant. Check whether the fall began on a specific date or slid gradually, because a cliff is an event and a slope is a season. Split the fall into traffic, conversion rate and order value, since seasonal falls almost always appear as traffic while conversion holds. And check that the current period is actually complete, because comparing twenty-two days against thirty produces a twenty-seven percent fall that is not real.
1. Compare with the same period last year
Seasonality repeats. If this September is down on August but up on last September, the business is growing and the month-on-month fall is the calendar.
This one check settles most cases, and it is the one people skip — because the previous month is what a dashboard shows by default.
2. Look at the shape, not the size
Plot the metric daily. A cliff on a specific date is an event: a tag broke, a payment method started failing, a campaign ended, a ranking dropped.
A gentle slope across weeks is seasonal or competitive. The shape tells you which investigation to run, and it is visible before you know the cause.
3. Split it into the three factors
Revenue is traffic multiplied by conversion rate multiplied by average order value. Work out each for both periods.
Seasonal declines almost always appear as traffic falling while conversion rate holds — fewer people are shopping, but the ones who do behave normally. A conversion rate that falls at the same time points at something on the site rather than something in the season.
4. Check the periods are actually comparable
- Is the current period complete? A month-to-date figure against a full month always looks like a collapse.
- Do both periods have the same number of weekends? Four weekends against five is not like-for-like for most businesses.
- Did a public holiday fall inside one and not the other? A single one can move a monthly total by several percent.
These are dull, and they explain a large share of the drops people spend a day investigating.